Bio-inspired nano-engineering of an ultrahigh loading 3D hierarchical Ni@NiCo2S4/Ni3S2 electrode for high energy density supercapacitors.

Energy density has become a critical barrier in supercapacitor engineering and improvement of the electrode-loading is urgently demanded. However, there is conflict between the high loading and good electrochemical properties of supercapacitors. Herein, ultrahigh loading (10.33 mg·cm-2) 3D hierarchical NiCo2S4/Ni3S2 on Ni foam with outstanding performance is obtained via bio-inspired nano-engineering, which contains compact nanowire arrays catching urchin-like micro-particles. Using this high-loading material as a binder-free electrode achieves excellent areal capacitances with 16.90 F·cm-2 at 10.33 mA·cm-2 and 1.17 F·cm-2 at 5.17 mA·cm-2 in a three-electrode system and asymmetric supercapacitor device, respectively. The device also exhibits a high energy density of 4.69 W h m-2 (power density of 10.33 W·m-2) and an outstanding stability of 91.4% after 8000 cycles (20.66 mA·cm-2). Its excellent performance is attributed to the well-designed structure and composition: (i) a large contact area with the electrolyte raises the utilization efficiency of the active material, therefore guaranteeing the high capacitance of the active materials; (ii) the high electronic conductivity network constructed through NiCo2S4 and the short diffusion length boost its rate performance; (iii) the reserved space in the hierarchical structure could hold the volume change and enhance the cycling performance of the electrode in the charge/discharge cycles. Thus, this work not only provides a method for the construction of a high-loading and high-performance electrode for asymmetric supercapacitors, but could also shed light on the design of compact nano-materials for other energy storage systems.